Set SubtitleView.mAlignment default value to be Alignment.ALIGN_CENTER

SubtitleView.mAlignment is null by default. This can cause undefined
behavior if mAlignment is passed to StaticLayout.Builder.setAlignment().
As the previous behavior of StaticLayout with alignment equals to null is
same as ALIGN_CENTER, this change uses ALIGN_CENTER as default value.

Bug: 119221721
Test: bit CtsWidgetTestCases:android.widget.cts.CheckedTextViewTest
Test: bit CtsWidgetTestCases:android.widget.cts.EditTextTest
Test: bit CtsWidgetTestCases:android.widget.cts.SwitchTest
Test: bit CtsWidgetTestCases:android.widget.cts.TextViewTest
Test: bit FrameworksCoreTests:android.widget.TextViewTest
Change-Id: Iefd069db449a7fe3d68f6f3e5ebcfcbf1e588962